There is used a thermal recording sheet in which information can be recorded by color development through a chemical reaction initiated by heating with a thermal head or the like. This thermal recording sheet is used as a recording medium in a facsimile, an automatic ticket vending machine, a scientific measuring instrument, or the like. Furthermore, the thermal recording sheet is also used as a recording medium for a POS system in a retail store. In this manner, this thermal recording sheet is used in a wide range of applications.
This thermal recording sheet often includes a printed layer formed by previously performing printing, other than a thermal recording layer that develops color by heating. For example, Patent Literature 1 discloses disposing a printed layer on the back surface side of a paper sheet that is a support body of the thermal recording body.
Hereinafter, as described herein, printing performed by allowing the thermal recording layer to develop color is referred to as “typing”, and printing having been previously performed is simply referred to as “printing.”
When a printed layer is formed on the back surface side of a paper sheet that is an opaque support body, like the technology disclosed in Patent Literature 1, the printed contents can be recognized only from the back surface side. Furthermore, the typed contents can be recognized only from the front surface. However, there are many cases in which both need to be recognized from the front surface side.
On the other hand, Patent Literature 2 discloses a technology of forming a printed layer on the front surface side of a thermal recording layer of a thermal recording body. The use of such a technology enables both the printed and typed contents to be recognized from the front surface side.
However, when a printed layer is formed on a thermal recording layer by printing, the thermal recording layer is concealed by the printed layer. Therefore, even if typing is performed, the typed contents cannot be recognized from the front view side. To address this concern, for achieving a structure in which both the printed and typed contents can be recognized from the front surface side, a thermal recording body needs to be configured such that a printed layer is not formed on the area where typing is required. The area where printing has been performed and the area where typing has not been performed, that is, the printed layer and the typing area, come to be formed in separate areas.
However, such separate areas cause a different thickness by as much as the typed layer. This leads to generation of a step between the printing area and the typing area. Accordingly, for example, when typing with a thermal head is performed, this difference in thickness causes optimum head adjustment to become difficult. Specifically, it becomes difficult to accurately adjust the distance between the head and the surface of a thermal recording sheet, and to adjust an optimum amount of heat. Furthermore, the step causes smoothness to decrease. Therefore, typing failures such as typing shrinkage and stick are likely to be caused.
Furthermore, when a printed layer is formed on the front surface, the printed layer is susceptible to scratches. In addition, a certain component contained in the printed layer may cause a thermal head to be contaminated by the printed layer or to fail.
Especially, when a printed layer is formed with metal-containing ink or the like, the surface of the printed layer is energized when it comes into contact with a thermal head. Then, the thermal head may fail by spark causing disconnection of the thermal head or by metal in the printed layer excessively absorbing heat energy. Furthermore, friction with the metal in the printed layer may cause the surface of the thermal head to wear. In addition, metal adhered to the thermal head may cause the surface of the thermal recording sheet to be scratched. Therefore, it is difficult to perform printing having excellent design features and metal gloss with the metal-containing ink.